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Earth Pressure
- Thread starter EireChch
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- #3
PEinc- that link tells me that k0 is based on poissons ratio? Is that how you derive it?
Also- do you consider different poisons ratios for drained and undrained condition? Does k0 changed if it’s drained or undrained ?
I am having a debate with a colleague and want to get a feel for what the engineers here think.
Also- do you consider different poisons ratios for drained and undrained condition? Does k0 changed if it’s drained or undrained ?
I am having a debate with a colleague and want to get a feel for what the engineers here think.
Retrograde
Structural
I am a structural engineer so take my advice for what it is worth.
For normally consolidated clay the k0 value is calculated the same as for a granular soil as (1-sin(phi)).
k0 is the same for drained and undrained.
For normally consolidated clay the k0 value is calculated the same as for a granular soil as (1-sin(phi)).
k0 is the same for drained and undrained.
geotechguy1
Civil/Environmental
For k0, either 1-sin(phi) or maybe generic published values for the soil type. Or if you know or measure in-situ horizontal and vertical effective stress, it's just the ratio. Poissons ratio is horizontal strain / vertical strain so not the same as K, one is about stresses and one is about strains.
K0 is the ratio of horizontal effective stress to vertical effective stress, at least as I understood it. I'm not sure k0 changes between drained and undrained conditions, but I suppose that the horizontal and vertical earth pressure might?
K0 is the ratio of horizontal effective stress to vertical effective stress, at least as I understood it. I'm not sure k0 changes between drained and undrained conditions, but I suppose that the horizontal and vertical earth pressure might?
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- #8
Thanks for the replies all. Dik - I have only ever known K0 to be related to phi angle, so it was news to me.
The reason I posed the question was that an engineer in my office was arguing that k0 = 1-Sin (phi) and that phi in undrained condition is 0 deg so K0 there for equals 1.
K0 is 1-Sin (phi'). It is effective friction angle. I have seen k0 = 1 for undrained condition in many many reports. and some of those reports have been reviewed and approved by some of the best in the business in the UK.
Its generally fine when the soils are normally consolidated as k0<1 in this instance, however when you get hihgly over consolidated clays, k0 can be as high as 2-3. So if you set k0=1 because the soil is in an undrained condition then you are dangerously wrong as you are under estimating the earth pressures.
The reason I posed the question was that an engineer in my office was arguing that k0 = 1-Sin (phi) and that phi in undrained condition is 0 deg so K0 there for equals 1.
K0 is 1-Sin (phi'). It is effective friction angle. I have seen k0 = 1 for undrained condition in many many reports. and some of those reports have been reviewed and approved by some of the best in the business in the UK.
Its generally fine when the soils are normally consolidated as k0<1 in this instance, however when you get hihgly over consolidated clays, k0 can be as high as 2-3. So if you set k0=1 because the soil is in an undrained condition then you are dangerously wrong as you are under estimating the earth pressures.
geotechguy1
Civil/Environmental
Completely speculation on my part but this thread was interesting:
Plaxis uses 0.5 as a default for undrained but apparently used to use 1 and had to change it because people were taking that as the value.
Then again I think I recall reading maybe in one of the ciria manuals that it's normal to take Ko as one for soils with higher OCRs because of construction-related effects (this would be for embedded type walls). I'll see if I can find it.
Plaxis uses 0.5 as a default for undrained but apparently used to use 1 and had to change it because people were taking that as the value.
Then again I think I recall reading maybe in one of the ciria manuals that it's normal to take Ko as one for soils with higher OCRs because of construction-related effects (this would be for embedded type walls). I'll see if I can find it.
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